The Cornea

What is the cornea?

The cornea is the transparent tissue that covers the front of the eye. An easy way to locate the cornea is simply to look at your eye in the mirror. You will notice a clear surface covering the iris (the colored part of the eye) and pupil. This is the cornea.

What is the function of the cornea?

Because the cornea is as smooth and clear as glass but as strong and durable as plastic, it helps the eye in two ways:

• The cornea provides a physical barrier that shields the inside of the eye from germs, dust, and other harmful matter. It shares this protective task with the sclera (the white of the eye).

• It acts as the eye's outermost lens. When light strikes the cornea, it bends -- or refracts -- the incoming light onto the crystalline lens. The lens then focuses the light onto the retina, the paper-thin tissue at the back of the eye that starts the translation of light into vision.

Although much thinner than the lens, the cornea provides about 65 percent of the eye's power to bend light. Most of this power resides in the center of the cornea, which is rounder and thinner than the outer part of the tissue and is thus better suited to bend lightwaves.

How important is the cornea to good vision?

The cornea is essential to good vision. As the eye's outermost tissue, the cornea functions like a window that controls the entry of light into the eye. For example, the cornea filters out some of the most damaging ultraviolet (UV) wavelengths in sunlight. Without this protection, the crystalline lens and the retina would be highly susceptible to injury from UV radiation.

If this "window" is curved too much, as is the case in some nearsighted people, faraway objects will appear blurry because distant light waves will refract imperfectly on the retina. If this "window" has imperfections or irregularities, as is the case in people with an astigmatism, light will refract unequally, causing a slight distortion of the visual image. But, if this "window" is of normal shape and curvature, light will refract with exquisite precision to the crystalline lens.

What is the structure of the cornea?

Although the cornea is clear and seems to lack substance, it is actually a highly organized group of cells and protein. The cornea receives its nourishment from the tears and aqueous humor that fills the chamber behind it. Unlike most tissues in the body, the cornea contains no blood vessels to nourish or protect it against infection. It must remain transparent to refract light properly, and the presence of even the tiniest capillaries would interfere with this process.

The tissue is arranged in three main regions, or layers.

EPITHELIUM. As the cornea's outermost region -- comprising about 10 percent of the tissue's thickness -- the epithelium functions primarily to: (1) block the passage of foreign material -- such as dust or water -- into the eye and other layers of the cornea, and (2) provide a smooth surface that absorbs oxygen and other needed cell nutrients that are contained in tears. This layer, which is about five cells deep, is filled with thousands of tiny nerve endings that make the cornea extremely sensitive to pain when rubbed or scratched.

STROMA. Located behind the epithelium, the stroma comprises about 90 percent of the cornea. It consists primarily of water (78 percent); layered protein fibers (16 percent) that give the cornea its strength, elasticity, and form; and cells that nourish it. The unique shape, arrangement, and spacing of the protein fibers are essential in producing the cornea's light-conducting transparency.

ENDOTHELIUM. This single layer of cells is located between the stroma and the aqueous humor. Because the stroma tends to absorb water, the endothelium's primary task is to pump excess water out of the stroma. Without this pumping action, the stroma would swell with water, become hazy, and ultimately opaque.

Each year, Medical College of Wisconsin physicians care for more than 180,000 patients, representing nearly 500,000 patient visits. Medical College physicians practice at Children's Hospital of Wisconsin, Froedtert Memorial Lutheran Hospital, the Milwaukee VA Medical Center, and many other hospitals and clinics in Milwaukee and southeastern Wisconsin.